1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) device employing a ZnO piezoelectric thin film, and more particularly, it relates to an improvement in a SAW device having such a structure that grating reflectors are arranged on both sides of an IDT (inter digital transducer).
2. Description of the Background Art
In recent years, mobile radio communication device, have been watched with interest. Table 1 shows frequency bands and necessary bandwidths (pass bandwidths/central frequencies) in such systems.
TABLE 1 ______________________________________ Frequency Specific 1.5 Times Band Bandwidth Specific System (MHz) (%) Bandwidth ______________________________________ Portable 800.about.900 2.7.about.3.8 4.1.about.5.7 Telephone of 800 to 900 MHz JDC of 1.5 1400.about.1500 1.6.about.1.7 2.4.about.2.6 GHz PHP 1900 1.3 2.0 DECT 1900 1.1 1.65 ______________________________________
Referring to Table 1, the meanings of the abbreviations of the systems are as follows:
JDC: Japan Digital Cellular PA1 PHP: Personal Handy Phone PA1 DECT: Digital European Cordless Telephone
As understood from Table 1, the specific bandwidth is at least 1% in every communication system. Therefore, an RF stage filter which is employed for a terminal of such a system must have a specific bandwidth of at least 1%, while its insertion loss in the band must be reduced to a considerably low level of about several dB.
An IIDT type (interdigitated interdigital type) SAW filter is widely employed as a filter having a wide band and low insertion loss. Such an IIDT type SAW filter is widely employed for the following reason:
As schematically shown in FIG. 1, a surface wave which is excited in an input side interdigital transducer (hereinafter referred to as "IDT") 1 is propagated to an output side IDT 2, where an output is taken out from the output side IDT 2. In this ordinary type of SAW with two IDTs, however, the surface wave is bidirectionally radiated from the IDTs 1 and 2. Thus, leakage signals are generated as shown by broken arrows in FIG. 1, causing a considerably large bidirectional loss of about 6 dB.
In a SAW filter having a three-electrode structure schematically shown in FIG. 2, on the other hand, output side IDTs 4 and 5 are arranged on both sides of an input side IDT 3. Therefore, no bidirectional loss is caused in the input side IDT 3. However, leakage signals are still generated outwardly from the output side IDTs 4 and 5 as shown by broken arrows in FIG. 2, causing a large bidirectional loss of about 3 dB.
In an IIDT type SAW filter 6 shown in a typical plan view of FIG. 3, however, a number of IDTs 8 are arranged on a piezoelectric substrate 7 along a surface wave propagation direction. According to this structure, first comb electrodes of the IDTs 8 are alternately connected to input and output ends IN and OUT along a surface wave propagation direction. In the IIDT type SAW filter, it is possible to reduce bidirectional loss by increasing the number N of the IDTs 8, thereby reducing energy leaking to the exterior of the SAW filter in the surface wave propagation direction.
Assuming that an IIDT type SAW filter is formed by (N-1)/2 input IDTs and (N+1)/2 output IDTs, i.e., N IDTs in total, bidirectional loss is expressed as follows: EQU 10log{(N+1)/(N-1)} . . . (1)
In a multielectrode filter having nine IDTs in total, for example, it is possible to reduce bidirectional loss to about 0.97 dB.
While it is possible to extremely reduce the bidirectional loss in the aforementioned IIDT type SAW filter as compared with the two IDTs type or three IDTs type SAW filter, however, the IIDT type SAW filter still has the loss of about 0.97 dB, which must be further reduced.
As shown in FIG. 4, therefore, reflectors 9 and 10 are arranged on outer sides of the plurality of IDTs 8 along the surface wave propagation direction, to substantially completely prevent leakage of surface wave energy from the outermost IDTs along the surface wave propagation direction. It is possible in theory to zero the bidirectional loss by arranging such reflectors 9 and 10.
FIG. 5 shows a vertical coupling type SAW resonator filter having a structure which has been recently studied with interest. This figure is a typical plan view showing only an electrode structure. Output side IDTs 12 and 13 are arranged on both sides of an input side IDT 11. Further, reflectors 14 and 15 are arranged on both sides of the IDTs 11 to 13. In this vertical coupling type surface wave resonator filter 11, it is possible to enlarge the band and reduce the loss, similarly to the aforementioned IIDT type SAW filter having the reflectors 9 and 10.
As hereinabove described, SAW filters are increasingly being developed with a structure wherein reflectors are arranged in some form on both sides of IDTS in the surface wave propagation direction, in order to enlarge the band and reduce the loss in the SAW filter.
In order to implement such a surface acoustic wave filter having a large band and small loss, a substrate material having a large electromechanical coupling factor is often employed. Typical examples of such a substrate are a 36.degree. Y-X LiTaO.sub.3 substrate and a 64.degree. Y-X LiNbO.sub.3 substrate. Table 2 shows material constants of these substrates.
TABLE 2 ______________________________________ V.sub.p (m/s) K.sup.2 (%) .alpha. (neper/.lambda.) ______________________________________ 36.degree. Y--X LiTaO.sub.3 4100 6.5 0.004 64.degree. Y--X LiTaO.sub.3 4450 10.4 0.009 (1120)ZnO/ 5200.about.5700 4.about.4.7 0.001 (0112).alpha.-Al.sub.2 O.sub.3 ______________________________________
Referring to Table 2, Vp represents phase velocities, K.sup.2 represents electromechanical coupling factors, and a represents attenuation coefficients in metal grating parts measured at 1 GHz, including those attenuated by radiation of bulk waves.
As clearly understood from Table 2, it is possible to increase the band by employing the 36.degree. Y-X LiTaO.sub.3 substrate or the 64.degree. Y-X LiNbO.sub.3 substrate having an extremely large electromechanical coupling factor K.sup.2. However, further reduction of the loss is inhibited by a relatively large attenuation coefficient .alpha..
On the other hand, Table 2 also shows material constants of a piezoelectric substrate (hereinafter referred to as a ZnO/.alpha.-Al.sub.2 O.sub.3 substrate) comprising a (0112) plane [0111] .alpha.-Al.sub.2 O.sub.3 substrate and a (1120) plane [0001] ZnO piezoelectric thin film formed on the substrate. As clearly understood from Table 2, the ZnO/.alpha.-Al.sub.2 O.sub.3 substrate has a large phase velocity Vp as well as a large electromechanical coupling factor K.sup.2. Thus, this substrate satisfies the requirements for a high acoustic velocity and a high coupling property, while also having an extremely small attenuation constant .alpha.. Thus, it is conceivably possible to readily reduce losses in a filter employing this substrate.
Thus, it is conceivably preferable to employ an IIDT type SAW filter provided with reflectors or a vertical coupling type SAW resonator filter utilizing a ZnO/.alpha.-Al.sub.2 O.sub.3 substrate as an RF stage filter for a terminal of each mobile communication device shown in Table 1.
In order to enlarge the pass band, however, it is necessary to widen stop bands of the reflectors, as a matter of course. While Table 1 shows specific bandwidths in the pass bands of the respective mobile communication devices, these specific bandwidths are mere minimum necessary values. In mass production, central frequencies and specific bandwidths are unavoidably dispersed, and frequency fluctuation caused by the ambient temperature etc. must also be taken into consideration. Therefore, bandwidths of the stop bands of the reflectors must conceivably be at least 1.5 times those shown in Table 1. Table 1 also shows values obtained by multiplying the specific bandwidths of the respective mobile communication devices by 1.5, for reference.
When the aforementioned substrate material is employed to form a SAW filter, therefore, the specific bandwidths of the stop bands of the reflectors are preferably at least about 1.5%, as clearly understood from Table 1.
In order to enlarge the specific bandwidths of the stop bands of the reflectors, admittance ratios of the reflectors may be increased, i.e., the reflectors may be increased in thickness. When the reflectors are increased in thickness, however, bulk wave radiation is increased in metal strip portions of the reflectors extending perpendicularly to the surface wave direction, to increase bulk wave conversion loss.
Also, the surface wave greatly attenuates due to applied mass of the metal strip, thereby causing an increase of the propagation loss, that is an, increase in the attenuation constant .alpha..
In consideration of the balance between the increase of the admittance ratios caused by an increase of the thicknesses, and the increase of the bulk wave radiation or the attenuation constant due to the applied mass, the specific bandwidths of the stop bands are inevitably restricted. Namely, it is necessary to sacrifice reduction of insertion loss to some extent, in order to enlarge the bands of the reflectors.
Thus, it is conceivably possible to form a SAW device having smaller loss as compared with other devices by employing a ZnO/.alpha.-Al.sub.2 O.sub.3 substrate having a small attenuation constant, while up to now there have been clarified no concrete conditions which can implement such a SAW device.